The changes in the mechanical properties of aluminum alloy bolts under high-temperature environments are an important research topic in materials science, focusing on the comprehensive influence of high temperatures on the microstructure and macroscopic mechanical behavior of aluminum alloys. At high temperatures, aluminum alloy bolts first exhibit a significant decrease in strength, a phenomenon closely related to the crystal structure characteristics of aluminum alloys. The strength of aluminum alloys mainly depends on precipitation strengthening, dislocation strengthening, and grain boundary strengthening mechanisms, which are disrupted by high temperatures. For example, precipitates coarsen or dissolve at high temperatures, weakening their resistance to dislocations; dislocation mobility increases with increasing temperature, dislocation entanglement decreases, and dynamic recovery and recrystallization processes accelerate, making the material more susceptible to plastic deformation; grain boundary strength also decreases at high temperatures, with grain boundary sliding becoming one of the main deformation mechanisms, further reducing the overall load-bearing capacity of the material.
The effect of high temperatures on the plasticity of aluminum alloy bolts exhibits a two-stage characteristic. In the intermediate temperature range, the plasticity of aluminum alloys usually increases. This is because the dynamic recovery process alleviates stress concentration caused by dislocation accumulation, allowing the material to withstand greater deformation. However, when the temperature exceeds a certain critical value, the plasticity actually decreases. This is because excessively high temperatures cause a sharp decrease in grain boundary strength, making it easy for voids to form and connect at the grain boundaries, ultimately leading to intergranular fracture. This shift in fracture mode causes the material to exhibit brittle fracture at high temperatures, with a fracture surface resembling a candy shell.
The elastic modulus of aluminum alloy bolts also changes significantly at high temperatures. As the temperature rises, atomic thermal vibrations intensify, and interatomic bonding forces weaken, leading to a linear decrease in the elastic modulus. This decrease not only affects the bolt's stiffness but also alters the stress distribution between the bolt and the connecting parts, thus affecting the overall structural stability. Especially under high-temperature vibration environments, the decrease in elastic modulus may cause the bolt to resonate, accelerating the fatigue failure process.
Microstructure evolution is the fundamental reason for the changes in the high-temperature mechanical properties of aluminum alloy bolts. At high temperatures, the grain structure of aluminum alloys undergoes significant changes. The dynamic recrystallization process may lead to grain refinement or coarsening, depending on the matching relationship between temperature and deformation. The evolution of precipitated phases is equally crucial; their coarsening, dissolution, or phase transformation directly weakens the strengthening effect. The dislocation configuration changes from a tangled state at room temperature to an ordered arrangement at high temperatures. While this transformation reduces dislocation density, it also makes the material more susceptible to slip deformation. Changes in grain boundary characteristics, such as grain boundary sliding and oxidation, become sensitive areas for failure at high temperatures.
To address the adverse effects of high-temperature environments on aluminum alloy bolts, various optimization measures are typically employed in engineering practice. Adjusting the alloy composition, such as adding elements like Sc and Zr, can refine the grains and increase the recrystallization temperature, thereby enhancing high-temperature stability. Optimizing heat treatment processes, such as using staged quenching or aging, can control the size and distribution of precipitates and delay the coarsening process at high temperatures. In terms of structural design, increasing the bolt diameter, reducing the thread helix angle, or using anti-loosening devices can improve the bolt's resistance to loosening at high temperatures.
The application of aluminum alloy bolts in high-temperature environments requires comprehensive consideration of the compatibility between material properties and operating conditions. For short-term high-temperature exposure scenarios, surface coatings or heat insulation measures can reduce bolt temperature; for long-term high-temperature service scenarios, specialized high-temperature aluminum alloy materials or nickel-based high-temperature alloy bolts must be selected. Regular inspection and maintenance are also key to ensuring the reliability of high-temperature bolts. By using non-destructive testing technology to detect cracks or loosening in a timely manner, catastrophic failures can be effectively avoided.
